Acoustic apparatus



Filed Aprii 20, 1935 Patented Mar. 6, 1934 UNITED STATES PATENT OFFICE Application April 20, 1933, Serial No. 667,103 In Great Britain May 21, 1932 11 Claims.

This invention relates to devices adapted to emit or receive sound-waves for the purposes of transmitting, recording and/or reproducing speech, music and other sounds. It is directed 6 especially to acoustic devices comprising a diaphragm mounted so as to have unrestrained movement at its periphery and loaded at its periphery by inertia rings or similar means whereby the frequency response characteristic of the 10 diaphragm is modified. A description of such a device will be found in British Patent Specification No. 332,272 filed on April th, 1929.

The invention consists in an acoustic device,

, such as a loud speaker or microphone, comprising a diaphragm loaded at its periphery by inertia rings or like loading means, wherein damping means are interposed between the diaphragm and the rings or like loading means.

The invention further consists in an acoustic device as set forth above wherein damping means are also provided externally of the inertia rings or like loading means.

The invention further consists in an acoustic device as set forth above comprising an electric coil or a driving unit attached to or mounted upon the diaphragm in a position removed from the centre of the diaphragm.

The invention further consists in an acoustic device as set forth above comprising an apertured shield positioned in front of the diaphragm and removed therefrom suiiiciently far to avoid acoustic damping thereby. This shield is preferably so far removed from the diaphragm surface that no damping of the diaphragm is effected thereby, and the aperture may form the neck of a horn preferably of exponential form.

Further features of the invention will become apparent from the following description of one form thereof which is however given merely by way of example and not as limitation to the scope of the invention specified above. This description will be more readily understood by reference to the accompanying figure representing in side sectional View the form of the invention 45 described.

In carrying the invention into effect in one convenient manner I provide a back-plate or spider a. of suitable material, adapted to support a diaphragm, b, preferably of metal, such as aluminum stretched across and in front of it, and held to the support a for example by suitably spaced pins 0 extending from the support through inertia rings d or like means positioned peripherally round and on each side of the dia- 55 phragm. These inertia rings are freely mounted upon the supporting pins 0, and have the effect of loading the periphery of the diaphragm whereby to modify the response characteristics thereof. Felt washers 0 may be provided against the inner and/or outer faces of the inertia rings at, for damping purposes.

Upon the rear face of this diaphragm is mounted a driving unit or a coil e whereby the diaphragm may be excited and/ or its vibrations due to incident soundwaves converted into electrical vibrations. This may comprise the usual form of such coil consisting in suitable insulated wire wound upon a former which is riveted or otherwise secured by a lip to the diaphragm, b, and it extends into the air gap of a pot magnet J sup ported by the back-plate or spider a. This may be either a permanent magnet or an electromagnet and may comprise for example a hollow central pole as shown extending through the backplate, upon which the energizing windings are mounted and covered by an outer cylindrical casing, forming the other pole, the poles being connected together at the outer ends by a rear plateyoke g, and the outer pole extending through suitable apertures in the diaphragm back-plate to support an annular pole piece 71. extending inwards towards the central pole to provide a suitably narrow air gap for the coil e.

The coil e is secured to the diaphragm b in a position slightly away from its centre and in one convenient arrangement may be displaced from central by 25% of the radius of the diaphragm. This has been found to render more uniform the efficiency of the diaphragm over a wide range of frequencies. In order still further to improve response damping rings or discs 2' of suitable material, e. g. cork, may be inserted between the diaphragm and the clamping or inertia rings (1 at its edge, these having the effect of damping or reducing considerably the amplitudes of vibrational waves across the diaphragm reflected from the edges.

According to a further feature of the invention the radiation efficiency of the system is increased by partially shielding the front of the diaphragm by a plate k of metal or other material having an aperture, concentric with the coil or driving unit 6 of area approximately equal to one quarter to one half the area of the diaphragm. This screen may be supported by a flange extending from the back-plate or spider, and is spaced well away (say inch away) from the face of the diaphragm so that substantially no damping effect is obtained. The aperture of the shield is may form the neck of a horn Z extending therefrom, preferably of exponential type, and may run into a baffle m of usual or known type.

It must be understood that the invention is not restricted to the various constructional details described above since many modifications may be introduced in order to carry the invention into effect under different conditions and requirements which have to be fulfilled without in any way departing from the scope of the invention.

Having now described my invention, what I claim as new and desire to secure by Letters Patent isz- 1. An acoustic device comprising a diaphragm loaded at its periphery and damping means interposed between said diaphragm and said peripheral load.

2. An acoustic device comprising a diaphragm loaded at its periphery by inertia rings and damping means interposed between said diaphragm and said rings.

3. An acoustic device comprising a diaphragm loaded at its periphery, damping means interposed between said diaphragm and said peripheral load and additional damping means contacting externally with said load.

4. An acoustic device comprising a diaphragm loaded at its periphery, damping means interposed between said diaphragm and said peripheral load and additional damping means contacting externally with said load, said diaphragm being mounted to allow peripheral movement free from restoring forces.

5. An acoustic device comprising a diaphragm loaded at its periphery, by inertia rings, damping means interposed between saiddiaphragm and said peripheral load and additional damping means contacting externally with said load, said diaphragm being mounted on pins extending through apertures in said rings.

6. An acoustic device comprising a diaphragm loaded at its periphery, damping means interposed between said diaphragm and said peripheral load and an electric coil attached to said diaphragm at a position oil-centre thereof.

'7. An acoustic device according to claim 6 wherein said coil is offset from the centre of said diaphragm by 25% of the radius of said diaphragm.

'8. An acoustic device comprising a diaphragm loaded at its periphery, damping means interposed cetween said diaphragm and said peripheral load, and an apertured shield positioned in front of said diaphragm but spaced therefrom to avoid acoustic damping of the diaphragm thereby.

9. An acoustic device according to claim 8 wherein the aperture of said shield is between one quarter and one half the area of said diaphragm.

10. An acoustic device comprising a diaphragm loaded at its periphery by inertia rings, damping means interposed between said diaphragm and said rings, an electric coil attached to said diaphragm in a position ofiset from the centre of said diaphragm, and a shield placed in front of said diaphragm, having an aperture between one quarter and one half of the area of said diaphragm concentric with said coil.

11. An acoustic device as claimed in claim 8 wherein said aperture forms the neck of a horn of exponential form.

ALBERT HENRY MIDGLEY. 

